fc-lang: gracefully handle the case where the last language initial is < 'z'
FcLangSetIndex() contains code like this:
low = fcLangCharSetRanges[firstChar - 'a'].begin;
high = fcLangCharSetRanges[firstChar - 'a'].end;
/* no matches */
if (low > high)
The assumption behind this test didn't hold before this commit, unless
there is at least one language name that starts with 'z' (which is
thankfully the case in our world :-). If the last language name in
lexicographic order starts for instance with 'x', this change ensures
that fcLangCharSetRanges['y' - 'a'].begin and
fcLangCharSetRanges['z' - 'a'].begin
are equal to NUM_LANG_CHAR_SET, in order to make the above assumption
correct in all cases.